This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-355136, filed Nov. 20, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a deformable mirror for an optical pickup, which records/reproduces information onto/from a recording medium. More particularly, the present invention relates to an optical pickup deformable mirror used for correcting a spherical aberration involved in a change in thickness of a cover glass of a recording medium.
2. Description of the Related Art
Presently, in the field of optical pickups, light having a wavelength of approximately 650 nm and an objective lens having a numerical aperture (NA) of approximately 0.65 are generally used.
In order to achieve higher-density recording, reduction in wavelength of the light to be used or increase in NA of the objective lens are carried on. For example, in the case of next-generation optical pickups, use of light having a wavelength of approximately 405 nm and use of an objective lens having an NA of approximately 0.85 are contemplated.
The thickness of the cover glass of an optical recording medium usually has irregularities of approximately xc2x13 xcexcm. Irregularities in the thickness of the cover glass generate a spherical aberration. The spherical aberration caused by irregularities of approximately xc2x13 xcexcm in the thickness of the cover glass is not a problem with conventional optical pickups and can be ignored, but it cannot be ignored with next-generation optical pickups working at a short wavelength or having a large NA, and the spherical aberration must be thus corrected.
Further, another technique for achieving higher-density recording involves realizing multiple information recording layers. A multilayered optical recording medium usually has a plurality of information recording layers, which are located at intervals of approximately 20 xcexcm.
For example, an optical pickup for a two-layered optical recording medium is optimally designed with reference to an intermediate position between the two information recording layers in order to minimize the spherical aberration generated. The optical pickup selectively reads/writes information from/onto the two information recording layers deviating from the optimum position by 10 xcexcm. This corresponds to reading/writing information from/onto a recording medium having a cover glass whose thickness deviates from an optimum value by 10 xcexcm.
Under such a condition, even if the conventional wavelength and NA are used, the spherical aberration caused by the difference in position of the information recording layers (in other words, the difference in thickness of the cover glass) can be no longer ignored, and hence correction must be carried out.
Use of a deformable mirror is proposed in order to correct the spherical aberration. The spherical aberration correction by the deformable mirror is disclosed in, for example, U.S. Pat. No. 5,202,875 or U.S. Pat. No. 5,719,846.
U.S. Pat. No. 5,719,846 teaches deformation of the mirror according to the expression (1), f(r)=3.3168xc3x9710xe2x88x922xc2x7r6xe2x88x923.9542xc3x9710xe2x88x923xc2x7r4xe2x88x920.505xc2x7r2, shown in the 23rd column. FIG. 6 shows a graph of this expression f(r). In the graph of FIG. 6, r is up to 2 mm, according to a description xe2x80x9cFurthermore, since the diameter of a light beam, which is subjected to the aberration correction, is about 4 mmxe2x80x9d in the 3rd column of the above-cited reference, and a unit of f(r) is mm.
Furthermore, this cited reference teaches deformation of the mirror according to the expression (2), f(r)=xe2x88x922.2xc3x9710xe2x88x921+3.4868xc3x9710xe2x88x921xc2x7r2xe2x88x921.277xc3x9710xe2x88x921xc2x7r4xe2x88x921.6348xc3x9710xe2x88x922xc2x7r6+7.0052xc3x9710xe2x88x923xc2x7r8, shown in the 23rd column. FIG. 7 shows a graph of this expression f(r). In the graph of FIG. 7, a constant term is omitted.
In U.S. Pat. No. 5,719,846, the mirror is deformed so as to cancel the optical path difference generated by the spherical aberration. That is, correction of the spherical aberration is attempted by reflecting a light ray incident upon the mirror so that an optical path difference proportional to the deformation of the mirror is introduced into the path of the light ray.
Deformation of the mirror not only corrects the optical path difference, but causes the direction of the reflected light ray to incline from the optimum direction. This inclination of the light ray moves the position of incidence upon the objective lens. Thus, the sufficient spherical aberration correction cannot be achieved by deformation of the mirror taking only the optical path difference into consideration.
U.S. Pat. No. 5,202,875 teaches, in the third paragraph of the 14th column, correction of the spherical aberration by using a Schmidt correction plate. However, this is also based on the principle similar to that of U.S. Pat. No. 5,719,846, and the spherical aberration can be corrected when the inclination of the reflected light ray is so small it can be ignored, but the sufficient spherical correction cannot be achieved when it is not.
According to the present invention, there is provided an optical pickup deformable mirror used for correcting a spherical aberration caused by a difference in thickness of a cover glass of a recording medium (irregularities in thickness of the cover glass at respective parts of the same recording medium, irregularities in thickness of the cover glass between different recording mediums, or the difference in cover glass thickness due to the switching of recording layers in a multilayered recording medium), wherein the optical pickup deformable mirror is deformed into a rotation-symmetrical surface obtained by rotating a curve represented by the following expression f(r):
f(r)=ar2+g(r)
where r is the distance from the axis of rotation symmetry, a is an actual number except for 0, and g(r) satisfies |g(r)|xe2x89xa61 xcexcm.
The optical pickup deformable mirror according to the present invention preferably satisfies |g(r)|xe2x89xa60.5 xcexcm.
The optical pickup deformable mirror according to the present invention more preferably satisfies |ar2|xe2x89xa60.005 mm.
The present invention provides an optical pickup deformable mirror, which can sufficiently correct a spherical aberration caused by a difference in thickness of a cover glass of a recording medium. The optical pickup deformable mirror according to the present invention can suppress not only generation of a spherical aberration but also occurrence of a comatic aberration by substantially deforming into a rotation-parabolic surface.
Additional advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.